This pair was recently (April, 2016) purchased for $202.50 plus $45 for shipping.  Winning the auction came as a surprise as my top bid was fixed. I really have no use for them; they caught my fancy as I haven't seen any since the sixties. Much to my surprise, they both work with the exception of one having a slightly rubbing voice coil.  Thinking it may be dirt, I drove the unit to about 5mm excursion in each direction to no avail.  The dust cap was removed using pure acetone and a slightly off center voice coil was noticed.  Shim stock, 0.015" was inserted around the pole piece and more acetone applied to the adhesive holding the coil to the apex of the cone.  It was then allowed to dry and set for about a day, after which the shims were removed.  the coil is now centered but slightly off parallel to the pole piece which isn't a problem.  Applying DC to the coil and observing the gap, the bottom of the coil slightly rubs the pole piece at about 6 mm outward excursion which is way beyond intended displacement, which is probably about 3mm in each direction.

One unit is marked as being a modified 602D duplex, which had a tweeter.  The hex head bolt is probably what was used to hold it to the pole piece.  The dust cap on this unit wasn't removed to observe the coil.  Results from subsequent tests indicate identical coils as well as reference made from an old (1986) Waldom parts catalog which lists identical voice coils.  It shows the same two coils for each of the 602 and 415.  They are the VC9300 and the VC3027.  Both have a coil form height of 15/16". diameter of 3.018" and winding length of 9/32".  The VC9300 has a dcr of 7.4 ohms while the VC3027 has a dcr of 5.5 ohms.  The former is a 16 ohm coil and the latter is 8 ohms. (impedances)  

These coils are wound with wire of circular cross section whereas the originals used by Altec are edge wound aluminum ribbon.  The unit marked as 415C has an underhung coil.  I can't say the same for the unit marked as a 602D.   I'd hazard a guess that the 602D has an overhung coil to allow for more excursion at the cost of efficiency.  Note in the Design photos the "no" (half space reference efficiency) difference of 0.5% and the BL product of the Design 2 being less than that of Design 1.  This is possibly due to an overhung voice coil in which about 1/6th of the coil windings are above and below the top plate.  The BL product is also less in the 602D than the 415C.  The BL product is the product of the field strength in the gap multiplied by the length of the wire in the gap, NOT the total length of the wire used in the coil.

Again, this is speculation as the coil in the 602D was not visually inspected and is also in contradiction to the replacement coils made by Waldom.

 

 

 

 

The left one was originally a 602D; the tweeter was removed for reason(s) unknown.  Note the two holes for the tweeter terminals which were removed.

There is also a 3/8 inch square label between the terminals on the 602D with "LF" on it, indicating low frequency.  If a similar label by the removed terminals existed, it was removed.

 

 

 

The left unit is the original 415C.  It not only had a rubbing coil but also a damaged annulus which can be seen at about 2 o'clock; the white stuff which is RTV, an uncured rubber available in a tube for about $7, available at most hardware stores.

The speakers were rotated as well as turned over to get better lighting.  These photos were taken without using a flash.  The left to right reversal of the units wasn't noticed until the writing of this page.

 

 

 

 

Near Field Responses

These curves are the near field responses of the two units.  The black is that of the original 415C and the red is the modified 602D.  These curves are derived from three, #1 taken at 1/8" above the dust cap, #2 at about 1.5 inches above the cap which is at the small "biflex" part of the cone and #3 at about an inch above the "biflex".  The three were then averaged to obtain those shown.  They are incredibly similar.

 

 

 

BiFlex A

Original 415C

The Thiele/Small parameters are also here.

The box volumes are optimized for a Qtc of close to 0.707.

While 7.5 ft^3 may seem large, the A7 Voice of the Theatre occupied about 17 ft^3

Here's an interesting story about the A7 or similar two way design that goes back to the 40's in Hollywood.  While listening to the soundtrack of a motion picture of a tap dancer, a double tap was noticed in lieu of a single; it sounded like an echo.  After some investigation, it was traced to the greater delay in the woofer vs the tweeter, caused by the larger mass of the woofer's moving system.  We now refer to it as phase shift or group delay.  By moving the tweeter further back, the double tap was eliminated.  That phenomenon is not unique to that system.  I've read arguments of similar phenomena with the Klipschorn.  The tweeter is only 2 inches long, if that and the midrange horn is about 18 inches and the bass horn is close to 48 inches.  I've done tests with a pulse generator in the regions of the crossover frequencies of 400 and 5000 hz and have not been able to detect an echo or double pulse.

Admittedly, the effect could have been masked by the reverberant field of the room.  Had the test been performed outdoors, the phenomenon may have been noticed.  A Hollywood sound stage is considerably larger than most houses, let alone an average sized room.

BiFlex B

Modified 602D

The box volume here has been reduced from the optimum of about 23 ft^3 for obvious reasons.  The difference in the low frequency response is negligible although a very critical ear may detect the difference in bass damping.  I.M.H.O., anyone that critical is paying more attention to the system than to the music.

 

The red and orange curves are for the biflex A and B in a closed box.  The yellow and green are for the same units in a reflex (ported) cabinet of 20 cubic feet.  

The vented cabinets have been reduced from the ideal of >50 ft^3 to a more tolerable size of 20 ft^3.  Note also that these Thiele/Small parameters pertain ONLY to these units which are at least 45 years old; the 415C was introduced in the mid fifties and is even older.  It was quite surprising to see its  parameters as good as they appear.  However, that is not to say anything about how it sounds.  I have a pair of corner cabinets fitted with Wharfedales.  They are about 6.5 cubic feet internally and these Altecs will be installed therein just to "see how they sound."

Those cabinets can be seen at the bottom of this PAGE.

 

Normalized Amplitude Response

Used to show the response curves shapes of each speaker in a box of given volume relative to another.

 

 

Custom Amplitude Response

Shows the sound pressure level at 1 meter for a specified power input, in this case, 50 watts.

 

 

Maximum Acoustic Power

Shows the maximum sound pressure level before the speaker reached either its mechanical or thermal limits, Xmax and Tvc, where Tvc is voice coil temperature. Usually, Xmax will be reached first and if maintained, Tvc will come into play. While modern adhesives may hold the moving system together, thermal runaway will rapidly result in total destruction. I have seen at a repair shop the result of some speakers that were used professionally and had actually burst into flame. 

It had to be quite an exciting sight to see.

 

 

Maximum Electrical Input Power

Shows how much power the speaker can handle prior to reaching either its mechanical or thermal limit.

 

 

Cone Displacement

Diaphragm excursion in one direction for a specified input power or voltage, assuming a constant voltage input. Since impedance usually rises with frequency, at least to resonance after which it drops, the power will drop if the voltage is held constant. 

P=E^2*cos a/Z, where a is the phase shift at the frequency used in the measurement.

 

 

System Impedance

Obviously shows the system impedance.

 

 

Phase Response

Gives the delay, in degrees between the time the input signal is applied to the voice coil and the pressure wave emanating from the speaker due to cone motion. 

An ideal situation would be a straight line at 0 degrees.

 

 

Group Delay

Similar to phase response but expressed in milliseconds. A uniform but modest delay isn't a problem. The smoother or flatter the delay, the better the transient response.  Note the high delay of the yellow and green curves for the 602D.  See the "interesting story" above under the Original 415C

 

 

 

 

Below is a link to a pdf file on the 412C and the 415C

www.voiceofthetheatre.com/images/412C.415C.1.jpg

 

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